Reference manager doesn't support multi-targeting

**Visual Studio Version**: 16

**Summary**:

If a project contains multiple target frameworks, the behaviour of the Reference Manager is determined only by the first in that list.

**Steps to Reproduce**:

1. Create a project with `netcoreapp2.0;net461`
2. Right click the _Dependencies_ node and click _Add Reference..._
3. Note the _Assemblies_ tab is not available
4. Reorder the `TargetFrameworks`
5. Repeat step 2
6. Note the _Assemblies_ tab is available

As @nguerrera said:

> Getting this fully right is hard and probably requires new UI, the references would have to be conditioned on tfm.